Radio frequency identification (RFID) systems and other forms of electronic article surveillance are increasingly used to track items whose locations or dispositions are of some economic, safety, or other interest. In these applications, typically, transponders or tags are attached to or placed inside the items to be tracked, and these transponders or tags are in at least intermittent communication with transceivers or readers which report the tag (and, by inference, item) location to people or software applications via a network to which the readers are directly or indirectly attached. Examples of RFID applications include tracking of retail items being offered for public sale within a store, inventory management of those items within the store backroom, on store shelving fixtures, displays, counters, cases, cabinets, closets, or other fixtures, and tracking of items to and through the point of sale and store exits. Item tracking applications also exist which involve warehouses, distribution centers, trucks, vans, shipping containers, and other points of storage or conveyance of items as they move through the retail supply chain. Another area of application of RFID technology involves asset tracking in which valuable items (not necessarily for sale to the public) are tracked in an environment to prevent theft, loss, or misplacement, or to maintain the integrity of the chain of custody of the asset. These applications of RFID technology are given by way of example only, and it should be understood that many other applications of the technology exist.
RFID systems typically use reader antennas to emit electromagnetic carrier waves modulated and encoded with digital signals to RFID tags. As such, the reader antenna is a critical component facilitating the communication between tag and reader, and influencing the quality of that communication. A reader antenna can be thought of as a transducer which converts signal-laden alternating electrical current from the reader into signal-laden oscillating electromagnetic fields or waves appropriate for a second antenna located in the tag, or alternatively, converts signal-laden oscillating electromagnetic fields or waves (sent from or modified by the tag) into signal-laden alternating electric current for demodulation by and communication with the reader. Types of antennas used in RFID systems include patch antennas, slot antennas, dipole antennas, loop antennas, and many other types and variations of these types.
In the case of passive RFID systems, the RFID tag is powered by the electromagnetic carrier wave. Once powered, the passive tag interprets the radio frequency (RF) signals and provides an appropriate response, usually by creating a timed, intermittent disturbance in the electromagnetic carrier wave. These disturbances, which encode the tag response, are sensed by the reader through the reader's antenna. In the case of active RFID systems the tag contains its own power source, such as a battery, which it can use to either initiate RF communications with the reader by creating its own carrier wave and encoded RF signals, or else the tag power can be used to enhance the tag performance by increasing the tag's data processing rate or by increasing the power in the tag's response, and hence the maximum distance of communication between the tag and reader.
Especially for passive RFID systems, it is often convenient to distinguish the behavior of RFID systems and their antennas in terms of near-field versus far-field behavior. “Near-field” and “far-field” are relative terms, and it is with respect to the wavelength of the carrier wave that the terms “near” and “far” have meaning. When the distances involved in an application are much greater than the wavelength, the application is a far-field application, and often the antenna can be viewed as a point-source (as in most telecommunications applications). On the other hand, when the distances involved in an application are much shorter than the wavelength, the relevant electromagnetic interactions between antennas (e.g., reader antenna and tag antenna) are near-field interactions. In such a situation the reactive electric or magnetic component dominates the EM field, and the interaction between the two coupled antennas occurs via disturbances in the field. When the application of interest involves distances on the order of the wavelength of the carrier wave, the situation is more complex and cannot be thought of as simply near-field or simply far-field. Below this situation will be termed “mid-field”.
Two common frequency bands used by commercial RFID systems are 13.56 MHz and UHF (approximately 850 to 960 MHz, with the specific band depending on the country in question). Since a tag on an RFID-tagged consumer item is generally used for many applications throughout the supply chain, from manufacturing and distribution to the final retail store location, the functional requirements of retail shelves are only one of the sets of factors influencing the choice of tag frequency. There are many factors and requirements of interest to various trading partners in the supply chain, and in this complex situation both 13.56 MHz and UHF are used extensively for tracking tagged items on and in smart shelving, racks, cabinets, and other retail, warehouse, and other business fixtures. U.S. Pat. Nos. 7,268,742, 6,989,796, 6,943,688, 6,861,993, 6,696,954, 6,600,420, and 6,335,686 all deal with RFID antenna applications to smart shelves, cabinets, and related fixtures. 13.56 MHz waves have a wavelength of just over 22 meters (72 feet), while the wavelength of UHF radiation used in RFID applications is approximately a third of a meter, or just one foot. Since the distances characteristic of item-level RFID applications involving the tracking and surveillance of tagged items on or in shelves, cabinets, racks, counters, and other such fixtures are on the order of feet (e.g., 0.5 fit to several feet), it is clear that, when UHF technology is used, the antenna interactions are neither near-field nor far-field, but rather are mid-field. In this case, a poor choice of reader antenna type, or the poor design of a proper type, can result in poor performance of the overall RFID system and application failure. One of the reasons for this is that in a mid-field situation the electric and magnetic fields emitting from the reader antenna vary significantly over the relevant surface (e.g., the surface of a retail shelf holding tagged items). The field may be strong in one place and much weaker in another place a few inches away (because the wavelength of UHF radiation is only a few inches), and the general behavior of the UHF system is much more complex than is observed in 13.56 MHz applications. Thus, in situations where UHF tags are used in RFID item tracking on shelves and other storage fixtures, the design of the reader antenna becomes critical. The current invention describes an approach to UHF antenna design which results in a uniform UHF emission zone immediately above the surface of the antenna (e.g., shelf surface) without large null (no-read) areas, and without requirement of a large antenna thickness which would limit the usefulness of the antenna design in practical retail and other business applications.
The detection range of passive RFID systems is typically limited by signal strength over short ranges, for example, frequently less than a few feet for passive UHF RFID systems. Due to this read range limitation in passive UHF RFID systems, many applications make use of portable reader units which may be manually moved around a group of tagged items in order to detect all the tags, particularly where the tagged items are stored in a space significantly larger than the detection range of a stationary or fixed reader equipped with one fixed antenna. However, portable UHF reader units suffer from several disadvantages. The first involves the cost of human labor associated with the scanning activity. Fixed infrastructure, once paid for, is much cheaper to operate than are manual systems which have ongoing labor costs associated with them. In addition, portable units often lead to ambiguity regarding the precise location of the tags read. For instance, the reader location may be noted by the user, but the location of the tag during a read event may not be known sufficiently well for a given application. That is, the use of portable RFID readers often leads to a spatial resolution certainty of only a few feet, and many applications require knowledge of the location of the tagged items within a spatial resolution of a few inches. Portable RFID readers can also be more easily lost or stolen than is the case for fixed reader and antenna systems.
As an alternative to portable UHF RFID readers, a large fixed reader antenna driven with sufficient power to detect a larger number of tagged items may be used. However, such an antenna may be unwieldy, aesthetically displeasing, and the radiated power may surpass allowable legal or regulatory limits. Furthermore, these reader antennas are often located in stores or other locations were space is at a premium and it is expensive and inconvenient to use such large reader antennas. In addition, it should be noted that when a single large antenna is used to survey a large area (e.g., a set of retail shelves, or an entire cabinet, or entire counter, or the like), it is not possible to resolve the location of a tagged item to a particular spot on or small sub-section of the shelf fixture. In some applications it may be desirable to know the location of the tagged item with a spatial resolution of a few inches (e.g., if there are many small items on the shelf and it is desired to minimize manual searching and sorting time). In this situation the use of a single large reader antenna is not desirable because it is not generally possible to locate the item with the desired spatial resolution.
Alternatively, a fully automated mobile antenna system can be used. U.S. Pat. No. 7,132,945 describes a shelf system which employs a mobile or scanning antenna. This approach makes it possible to survey a relatively large area and also eliminates the need for human labor. However, the introduction of moving parts into a commercial shelf system may prove impractical because of higher system cost, greater installation complexity, and higher maintenance costs, and inconvenience of system downtime, as is often observed with machines which incorporate moving parts. Beam-forming smart antennas can scan the space with a narrow beam and without moving parts. However, as active devices they are usually big and expensive if compared with passive antennas.
To overcome the disadvantages of the approaches described above, fixed arrays of small antennas are utilized in some UHF RFID applications. In this approach numerous reader antennas spanning over a large area are connected to a single reader or group of readers via some sort of switching network, as described for example in U.S. Pat. No. 7,084,769. Smart shelving and other similar applications involving the tracking or inventory auditing of small tagged items in or on RFID-enabled shelves, cabinets, cases, racks, or other fixtures can make use of fixed arrays of small antennas. In tracking tagged stationary items in smart shelving and similar applications, fixed arrays of small antennas offer several advantages over portable readers, systems with a single large fixed antenna, and moving-antenna systems. First, the antennas themselves are small, and thus require relatively little power to survey the space surrounding each antenna. Thus, in systems which query these antennas one at a time, the system itself requires relatively little power (usually much less than 1 watt). By querying each of the small antennas in a large array, the system can thus survey a large area with relatively little power. Also, because the UHF antennas used in the antenna array are generally small and (due to their limited power and range of less than 1-12 inches) survey a small space with a specific known spatial location, it must also be true that the tagged items read by a specified antenna in the array are also located to the same spatial resolution of 1-12 inches. Thus systems using fixed arrays of small antennas can determine the location of tagged items with more precision than portable RFID readers and systems using a small number of relatively large antennas. Also, because each antenna in the array is relatively small, it is much easier to hide the antennas inside of the shelving or other storage fixture, thus improving aesthetics and minimizing damage from external disruptive events (e.g., children's curiosity-driven handling, or malicious activity by people in general). Also, an array of fixed antennas involves no moving parts and thus suffers from none of the disadvantages associated with moving parts, as described above. Also, small antennas like those used in such antenna arrays may be cheaper to replace when a single antenna element fails (relative to the cost of replacing a single large antenna). Also, fixed arrays of antennas do not require special manual labor to execute the scanning of tagged items and, therefore, do not have associated with them the high cost of manual labor associated with portable reader and antenna systems, or with mobile cart approaches.
In smart shelving and similar applications it is often important for economic and aesthetic reasons that the antennas used in the antenna array be simple, low cost, easy to retrofit into existing infrastructure, easy to hide from the view of people in the vicinity of the antennas, and that the antennas can be installed and connected quickly. These application requirements are more easily met with an antenna configuration which minimizes the number of layers used in the antenna fabrication, and which also minimizes the overall antenna thickness. That is, thin or low profile antennas are easier to hide, and easier to fit into existing infrastructure without requiring special modification to that existing infrastructure. Also, reducing layers in the antenna tends to reduce antenna cost. For reasons of cost and installation convenience it is also desirable to have the simplest possible approach to the attachment of the RF feed cables or wires to the antennas. Preferably, the attachment should be made in one location, on one surface, without requiring a hole or special channel, wire, or conductive via through the antenna substrate. This last requirement is especially important in large-volume manufacture of the antenna systems since, in that case, the final assembly will usually involve a few hand assembly steps carried out by an electronics technician on an assembly line, and elimination of one or several steps will significantly reduce the total production cost. It is also important that the design of the UHF antennas allows for reading of RFID tags in the space near the antennas without “dead zones” or small areas between and around antennas in which the emitted fields are too weak to facilitate communication between the tag and reader. Another requirement for the antennas used in smart shelf and similar applications is that they have the ability to read items with a diversity of tag antenna orientations (i.e., tag orientation independence, or behavior at least approaching that ideal).
Traditional patch antennas, slot antennas, dipole antennas, and other common UHF antenna types which might be used in antenna systems such as those described above generally involve multiple layers. U.S. Pat. No. 6,639,556 shows a patch antenna design with this layered structure and a central hole for the RF feed. U.S. Pat. No. 6,480,170 also shows a patch antenna with reference ground and radiating element on opposing sides of an intervening dielectric. A multi-layer antenna design can lead to excessive fabrication cost and excessive antenna thickness (complicating the retrofitting of existing infrastructure during antenna installation, and making it more difficult to hide the antennas from view). Multi-layer antenna designs also tend to complicate the form of the attachment of the connecting wires (for example, co-axial cable between the antenna and reader) since the connections of the signal carrier and reference ground occur on different layers, and this increases the cost of the antenna for the reasons described above.
For UHF smart shelving applications the patch antenna is a good choice of antenna type because the fields emitted from the patch antenna are predominantly in the direction orthogonal to the plane of the antenna, so the antenna can be placed on or inside the shelf surface and create an RFID-active space in the region immediately above the shelf, and read the tagged items sitting on the surface of the shelf with relative ease. Of course, this presupposes that the particular patch antenna design yields sufficient bandwidth and radiation efficiency to create, for a given convenient and practical power input, a sufficiently large space around the antenna wherein tagged items can be dependably and consistently read. The traditional patch antenna described in the prior art has a main radiative element of conductive material fabricated on top of a dielectric material. Beneath (i.e., on the reverse side of) the dielectric material is typically located a reference ground element, which is a planar layer of conductive material electrically grounded with respect to the signals being transmitted or received by the antenna. In the typical patch antenna design well known in the prior art, the antenna main radiative element and the reference ground element are in parallel planes separated by the dielectric material (which, in some cases, is simply an air spacer). Also, in the usual case, the main radiative element and the reference ground element are fabricated with one directly above the other, or with one substantially overlapping with the other in their respective parallel planes. A disadvantage of this traditional multi-layer patch antenna design is that the connection of the shielded cable or twisted pair wire carrying signals between the antenna and the RFID reader must be attached to the antenna on two separate levels separated by the dielectric material, thus requiring a connecting hole or via in the dielectric layer.
The size of the gap between the radiating element and the reference ground conductor (i.e., the dielectric layer thickness) is a critical design parameter in the traditional patch antenna since, for a given dielectric material, the thickness of this gap largely determines the bandwidth of the antenna. As the gap is reduced, the bandwidth is narrowed. If the bandwidth of the antenna is too narrow, the tuning of the antenna in a given application becomes very difficult, and uncontrollable changes in the environment during normal operation (such as the unanticipated and random introduction of metal objects, human hands, or other materials into the area being monitored by the antenna) can cause a shift in resonance frequency which, combined with the overly narrow bandwidth, causes failure in RFID tag detection and reading. Thus, for a given application there is for practical reasons a lower limit on the distance between the ground plane and the radiating element in a traditional patch antenna design, and this constrains the overall thickness of the antenna.
Another constraint on the thickness of a traditional patch antenna stems from radiation efficiency (fraction of total electrical energy put into the antenna which is emitted as electromagnetic radiation). If the dielectric thickness or gap between the reference ground and radiating element is too small, the radiating efficiency will be too low, and too much of the power to the antenna is wasted as heat flowing into the dielectric and surroundings.
The discussion above makes it clear that (1) a patch antenna design can be used effectively in UHF smart shelf and similar applications, and (2) use of the patch type of antenna would be even more advantageous, and satisfy the previously discussed practical requirements of smart shelving more completely if there were some way of overcoming the constraints on the thickness of the antenna imposed by the requirements of high bandwidth and radiation efficiency. Also, it would be advantageous to find a new design for the patch antenna which simplifies the attachment of the feed cable or wire. In addition, it would be advantageous to find a new antenna design which spread the UHF radiation more evenly and over a greater area of the surface of the shelf containing the antenna (i.e., in the region above the radiating element plane) than is possible for the traditional patch antenna design. As noted above, the relatively short wavelength (approximately 12 inches) of UHF emissions can present challenges to the designers of UHF smart shelving who want to be able to effectively and consistently read tags at any location on the shelf. A better UHF antenna design would minimize this problem, and allow better “field spreading” or “field shaping” in the regions immediately above and around the edges of the antenna.
The current invention overcomes the above-mentioned limitations of the traditional patch antenna design, and results in a new patch antenna which is much thinner without sacrificing bandwidth and radiation efficiency. Also, the current invention allows for a much more simple antenna feed cable attachment than is possible with the traditional patch antenna approach. Also, the current invention allows for a more evenly distributed UHF field around the antenna which makes it easier to avoid dead zones, and allows the smart shelf designer to spread or shape the field evenly around the antenna. In contrast to this prior art, the current invention describes an antenna in which the main radiative element is placed in a common geometric plane, or substantially the same plane, with the reference ground element, or in which the main radiative element and reference ground element are placed in two parallel, closely spaced planes separated by a dielectric laminate, with little or no overlap between the main radiative element and the reference ground element. That is, a key invention described in this specification is a patch antenna in which the main radiative element and the reference ground element are in the same plane, or in two closely-spaced parallel planes, with the two elements substantially side-by-side rather than one directly over the other, or rather than one substantially overlapping with the other. This cost-efficient antenna configuration, particularly when implemented with a floating ground plane or planes in addition to the reference ground element, and with the floating ground plane or planes located beneath the plane holding the main radiative element and reference ground, results in superior antenna gain, bandwidth, and tuning robustness in RFID smart shelf applications, as well as similar applications in which it is desired to interrogate a number of RFID tags located in close proximity, with low-power RFID signals localized in a small physical space which would normally result in tuning difficulties for traditional patch antennas. A further advantage of the current invention is that the newly invented patch antenna is thinner than a typical patch antenna described in the prior art. That is, by locating the main radiative element and the reference ground element in the same plane, or substantially the same plane with little or no overlap, a thinner patch antenna can be designed for a given high bandwidth, radiative efficiency, and robust frequency response requirement.